Once again Sergio Agüero was the man who condemned Manchester United. The elegant swish of his right boot that won this match will have far less significant consequences than its more famous counterpart against QPR last May but Manchester City will still cherish its impact and the message it sends to the team who are in the process of removing the championship trophy from their possession.

The gap is now 12 points with seven games to play and it means that wonderful moment when Agüero took Yaya Touré's pass, eluded three United players with his combination of balance and power and speed, then thumped his shot into the roof of David de Gea's net, will probably be only a consolation prize come the end of the season. Yet here was the evidence that there is still very little between these sides despite the gulf that has opened up at the top of the league. For all their elation the boisterous corner of City supporters must wonder why their team could not have exhibited this drive and togetherness more often. They had played like true champions.

Both sides gave everything but the team in blue had the greater penetration and sense of purpose. United will comfort themselves with the knowledge that 10 more points will still confirm their 20th league title. They can look ahead with confidence and circle their visit to Arsenal on 28 April as a possible date for the coronation but they will have found this game a disconcerting experience.

Sir Alex Ferguson described Robin van Persie's performance as "fantastic" but it was a generous appraisal and the Dutchman's blip is in danger of becoming a slump. Wayne Rooney was substituted in a match when his only memorable contribution was a dangerous studs-up challenge on James Milner. Ashley Young was ineffectual before his injury and Michael Carrick struggled to exert his usual influence. As for Ryan Giggs, he was the first man off the pitch at the end, straight down the tunnel with the look of a man who had just found someone had run a key down the side of his car. There was something deeply incongruous about a player of this experience and football intelligence contributing to City's opening goal with a senseless back-heel inside his own half.

That was six minutes into a second half in which City looked the fitter, stronger team and United's efforts started to appear increasingly desperate. Gareth Barry had read Giggs's intentions and Milner's left-foot shot, from Samir Nasri's lay-off, took a deflection off Phil Jones to find its way past De Gea's stretching right hand.

Milner had been excellent on the right side of City's attack but was far from alone. Was this the same Nasri who had been operating for most of the season, by Mancini's own reckoning, at 50%? Matija Nastasic reminded us why he has displaced Joleon Lescott while Gaël Clichy eventually got the better of Danny Welbeck and Vincent Kompany was more like the player we saw at the end of last season.

City's defence, on the whole, protected Joe Hart well, which is probably just as well judging by his contribution when Kompany's own-goal brought United level just before the hour. Hart had completely misjudged the trajectory of Van Persie's free-kick and Jones, at the far post, really ought to have scored. Instead his attempted header struck him on the shoulder and ricocheted off the back of Kompany into the exposed net.

It had been a frenetic night, two sides slugging it out from one end to the other during the first half, often abandoning normal protocol to line up 4-2-4, attack versus defence. Mancini's team had begun the match as though determined to show the gulf in points was not a true reflection of the sides. United, in turn, set off like a side impatient to be reunited with the championship trophy. What neither team could do in the first half was create the clear chance. With the game at that pace, it was probably inevitable a little refinement would be missing.

There was an edge as well. Mancini could be seen haranguing the referee, Mike Dean, as they left the pitch at half-time for blowing the whistle just as City were putting together a promising attack. Later Agüero was incensed when Rio Ferdinand opted to play on despite David Silva being injured. Giggs and Barry started grappling in the subsequent mêlée and it took some time for Dean to restore order.

Ferguson's assessment was that United had been the better team but it felt like a deception. After 79 minutes Yaya Touré slipped the ball into Agüero just inside a congested penalty area. Agüero, a second-half substitute, found a gap and continued running, left to right and away from Welbeck, Jones and Ferdinand with one change of movement and acceleration. His shot was a beauty, still rising as it hit the net.